Biomedical subjects
Publications and source records attributed to J S Pointer.
The contrast sensitivity gradient across the major oblique meridians of the visual field is compared with previous estimates of the horizontal and vertical meridians.
The regional variation of contrast sensitivity along the greater extent of each of the four principal hemi-meridia of the normal human eye was determined under photopic conditions using horizontally-orientated sinusoidal grating stimuli. The stimuli were well localized in space and frequency, and special attention was paid to the low spatial frequency range. The results confirm that contrast sensitivity is maximal for central vision for all test spatial stimuli. Extra-foveal fall-off in sensitivity can be represented as a linear function of eccentricity if the latter is expressed in relative units (i.e. periods of the stimulus). The regional variation parameter depends upon whether the horizontal or vertical field is tested and upon the spatial frequency of stimulation. The visible spatial frequency range (0.05-24 c/deg) can be approximately described by just three different rules. The fact that more than one rule is found bears upon current models of the functional organization of the visual system.
Spatial and temporal contrast sensitivity was measured, under identical test conditions using spatially and temporally localized stimuli, in 4 normally sighted human subjects and in 3 healthy hemianopic subjects whose stable visual field loss exhibited foveal sparing. Testing was undertaken at central fixation and at a variety of eccentric loci along the horizontal meridian of the sighted and perimetrically blind field. In comparison with the normal (control) sensitivities obtained in this study, spatial and temporal sensitivities in the sighted hemifield in subjects with striate cortical lesions were both reduced; in addition, we were unable to demonstrate greater-than-chance performance at any extrafoveal locations in the blind hemifield.
The information used in the output of the visual filters was examined for a blur discrimination task for which it had previously been claimed that the best human performance was with pedestal blurs of intermediate magnitude. This conclusion was verified for parameters leading to asymptotic performance for foveal and parafoveal locations. The nature of the computations underlying these visual discriminations was then assessed experimentally and by simulations. The results suggest that a frequency-based model in its simplest form cannot explain these findings. A model based on the spatial properties of filter outputs is discussed.
A televised discrimination task was used to assess visual performance with spectacles and then with hydrophilic ('soft') contact lenses in a small group of human subjects. Concurrent recording of eyeblink activity indicated that an increased blink rate recorded during the first month of soft contact lens wear was confined to the immediate post-response phase of testing. Furthermore, as adaptation to lens were proceeded, the blink pattern was revealed as being refined to visual task difficulty.
The results of recent investigations of supra-threshold spatial vision in amblyopia have indicated that the amblyopic eye may be more disadvantaged than acuity or contrast detection measurements have suggested. We report here on the sensitivity of the amblyopic eye when performing tasks requiring the recognition of small spatial differences in thin bright line stimuli displayed in a variety of 2-D configurations. Amblyopes were able to perform these tasks, but their amblyopic eye was generally poorer than their normal eye by a multiplicative factor. It is argued that this pattern of results indicates that spatial scrambling rather than blurring is an appropriate model for amblyopia.
Human visual sensitivity for discriminating between, on the one hand stimuli composed of components (F) and (F + 3F) (compound detection), and on the other hand (F + 3F) and (F - 3F) (phase discrimination), was measured as a function both of stimulus contrast and eccentricity. Performance under these particular conditions was found to depend upon whether the (F) or (3F) component was dominant in the pattern. When the (F) component was high in contrast, visual performance was well modelled by an edge-blur discrimination, whereas when the (3F) component was high in contrast, visual performance was well modelled by a contrast discrimination involving local spatial features within each waveform. These conclusions were valid for both foveal and peripheral vision. The finding that these suprathreshold compound stimuli are discriminated on the basis of the local spatial features, and not on differences in their phase spectra as previously thought, allows a reinterpretation of the importance of phase coding in normal vision and of the selective loss of these discriminations that have been previously reported for peripheral vision and in amblyopia.
1. Detection thresholds for two-dimensional Gabor functions of varying spatial and temporal frequency were used to investigate the post-receptoral sensitivity across the retina of the typical and complete achromat. 2. Under photopic conditions there is no evidence for post-receptoral cone function at any retinal eccentricity investigated. Sensitivity saturates in a way consistent with known psychophysical and electrophysiological measures of rod saturation. This occurs in a unitary fashion across the retina. 3. Under scotopic conditions the regional fall-off in spatio-temporal sensitivity is similar for the achromat and duplex retina. This suggests that the rods in the achromat make normal neural connections. 4. Taken together this supports the contention that the typical and complete achromat is a functional rod monochromat and hence can be used to explore the sensitivity of the isolated rod post-receptoral mechanism under mesopic conditions where its sensitivity is optimal. This is where its contribution is most difficult to isolate in the duplex retina. 5. For the human rod mechanism, mesopic post-receptoral sensitivity for all spatio-temporal stimuli is optimal in the central region of the retina and falls off as a function of eccentricity. 6. For localized stimuli, peripheral spatial sensitivity is reduced evenly at all spatial frequencies compared with that of the central retina. A similar displacement of the spatial sensitivity function of the rod mechanism occurs as illuminance is reduced. 7. For localized stimuli, temporal acuity of the rod mechanism is around 20-25 Hz irrespective of retinal position. As the illuminance is further lowered dynamics of the rod pathway are reduced irrespective of retinal position and the sensitivity function maintains a bandpass shape. 8. The regional distribution sensitivity of the rod mechanism changes as illuminance is reduced from mesopic to scotopic levels.
A subjective guessing bias has been claimed to be present in association with visual acuity testing using the classic circular Landolt ring. A semiautomated visual test system has been described previously which uses an angular version of the broken ring. Evidence is presented here which indicates that this version of the test object overcomes the problem of guessing bias and thus might prove to be not only a preferred "standard" or reference optotype but also an acceptable clinical optotype in its own right.
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The cerebral representation of peripheral sensory surfaces is strictly ordered. In the case of the visual system, the scale of the cortical representation of visual space is non-linear: the area devoted to the fovea is greatly enlarged. Attempts have been made to quantify this retino-cortical relationship in animals and man: the result is the cortical magnification factor (M) which indicates the linear distance in mm along the striate cortex concerned with each degree of the visual field. The cortical magnification factor has indicated an alternative approach both to the theoretical interpretation of visual processing and to the clinical aspects of visual field examination. By M-scaling vision test stimuli, visual sensitivity, while quantitatively different at foveal and peripheral projections, remains qualitatively similar across the entire field.
Spatio-temporal contrast sensitivities to horizontally-oriented Gaussian-weighted patches of sinusoidal grating stimuli were determined across the nasal and temporal visual fields of strabismic and non-strabismic, anisometropic amblyopes. The visual field distribution of the amblyopic anomaly differs in strabismic and non-strabismic, anisometropic eyes. In strabismus the peripheral region of one or both hemifields is spared; in non-strabismic, anisometropic cases the loss is evenly distributed across the binocular visual field but is not present in the monocular temporal field. These findings suggest that the non-strabismic forms of amblyopia in humans result from binocular competitive imbalance in early life. The strabismic results pose two problems for the present competitive model of amblyopia: in strabismus, amblyopia is mainly limited to central vision and shows an asymmetric distribution.
An instability of the visual correction is sometimes associated with soft hydrophilic contact lenses. However, this problem does not seem to have detracted from the commercial success of this form of refractive aid. To facilitate the quantification of visual performance during soft lens wear, a televised test system was developed and has been fully described elsewhere. Results are now presented of a clinical study using this television (TV) system. Visual performance scores, in conjunction with response latency and blink activity data, were obtained from the eyes of 27 experimentally naïve subjects during the first month of adaptation to soft contact lens wear. The results suggest that the soft contact lens wearer has recourse to a strategy whereby he can optimize his level of visual performance when necessary.